The aim of this paper is to describe XML as one of the most important markup languages, to explain its application and to emphasize the importance of using XML in libraries in the modern information environment. The first part of the paper describes the markup language XML, its history and a brief overview of the languages that preceded it, the basic concepts and syntax of XML, its technologies, and applications with corresponding examples. Also, comparison was made with other markup languages, with emphasis on HTML. After the description of XML, there is a description of two selected standards written in the markup language XML, i.e., MARCXML and MODS, which are mostly used in libraries when digitizing library materials. First is described MARCXML, its design, schema, and implementation. Then is described MODS and its scheme, application, and design. In addition, the paper emphasizes the role that the application of XML has today in the library, that is, in the process of digitization of library materials where XML plays an important role as a suitable format for serialization, exchange and management of metadata in the library. The description of the MARCXML and MODS standards is later followed by their practical examples and their comparison as well. There are also listed the advantages and positive sides of their use as well as the disadvantages that users encounter when using these standards.